1. Field of the Invention
The present invention relates to a frequency shift keying (FSK) transceiver. Such circuits are generally used to transmit, especially by radio, digital data.
2. Discussion of the Related Art
The drawing schematically shows an FSK transceiver of a type usable according to the present invention. The transmit portion Tx and the receive portion Rx are integrated in the same circuit.
The transmit portion includes a phase-locked loop 10 used to obtain an adjustable carrier frequency. This phase-locked loop includes a reference oscillator 12, the output signal of which, of fixed frequency, is provided to a phase comparator 14. Phase comparator 14 receives the output of a controlled oscillator 16 via a divider 18. Oscillator 16 is controlled by a low-pass filter 20 that receives the output of phase comparator 14.
The binary signal to be transmitted Txin is received on a pin 22 of the integrated circuit. Signal Txin is added to the control signal of oscillator 16 at 24 via an amplifier 26. The gain of amplifier 26 determines the xe2x80x9cdeviationxe2x80x9d of the FSK modulation, that is, the frequency shift undergone by the carrier provided by oscillator 16 when signal Txin switches between logic levels 0 and 1. This deviation must in some applications be precisely determined. For this purpose, the gain of amplifier 26 is adjustable, for example, externally by means of an adjustable resistor 28 connected to a pin 30 of the circuit.
The output of oscillator 16 provides the modulated signal to be transmitted Txout on a pin 32, possibly via an amplifier 34.
In receive portion Rx, a received modulated signal Rxin, provided for example by an antenna 36, is applied on a pin 38 of the integrated circuit. An amplifier 40 transmits this signal to two mixers 42 and 43. The two mixers also respectively receive two carriers of same frequency in phase quadrature.
Since the circuit is generally used in a single way, either to transmit, or to receive, the controlled oscillator 16 of phase-locked loop 10 is used in receive mode to provide the two carriers in phase quadrature. Then, no modulating signal is provided to the phase locked-loop, so that oscillator 16 operates at the frequency defined by reference oscillator 12 and divider 18, which frequency is no other than the carrier frequency.
The outputs of mixers 42 and 43 are provided to respective low-pass filters 45 and 46. Two signals in phase quadrature, the frequency of which is that of the above-mentioned deviation, are thus obtained. Further, state 1 corresponds to a 90xc2x0 phase advance of the first channel with respect to the other, and state 0 corresponds to a 90xc2x0 phase lag of the first channel with respect to the other.
The output signals of filters 45 and 46, of sinusoidal shape, are provided to respective clipping amplifiers 48 and 49 intended for converting these signals into rectangular signals. These rectangular signals are provided to an FSK demodulator 51 that extracts logic values 0 or 1 by analyzing the phase of the signals provided by clipping amplifiers 48 and 49. Demodulator 51 provides the binary extracted signal Rxout to a pin of the integrated circuit.
Generally, since the number of circuit pins is high and the circuit is used in a single way, signal Rxout may be provided, as shown, to pin 22 which is used, in transmit mode, to receive signal Rxin to be transmitted. A switch K enables choosing the required function of pin 22.
As previously mentioned, the gain of amplifier 26 must be precisely adjusted to obtain a correct deviation in the modulation of the transmitted signal. For this purpose, the circuit is set to the transmit mode, a test signal is applied on pin 22, and the output signal sampled from pin 32 is analyzed. The analysis generally consists of performing a frequency demodulation followed by a spectrum analysis. Resistor 28 is adjusted until the spectrum corresponds to the desired deviation. This adjustment solution requires particularly expensive laboratory equipment.
An object of the present invention is to provide a solution enabling adjustment of the deviation with low cost laboratory equipment.
To achieve this and other objects, the present invention provides an FSK transceiver circuit, including a controlled oscillator used, in a transmission mode, to modulate a carrier by frequency shifts according to a binary transmission signal present on an input pin of the circuit, the shift amplitude being determined by an adjusting element connected to an adjusting pin of the circuit; two mixers connected to receive a reception signal present on a receive pin and, respectively, two carriers in phase quadrature of the controlled oscillator that, in receive mode, receives no modulating signal; and a demodulator connected to the outputs of the mixers via respective filters, the output of one of the filters being accessible from the outside by a circuit test pin.
According to an embodiment of the present invention, the controlled oscillator is part of a phase-locked loop.
The present invention also aims at a method for adjusting a circuit as described hereabove, including the steps of:
setting the controlled oscillator to the transmit mode;
applying a carrier on the receive pin;
applying a binary test signal on the input pin;
measuring the frequency of the signal present on the test pin; and
acting upon the adjusting element to obtain the required frequency of the signal present on the test pin.